🔇 How it works
Sound (air vibration) hits the porous walls (mineral wool) and converts acoustic energy into heat. The air exits unchanged - the noise stays behind. Typical attenuation: 15-30 dB depending on length and design.
Sound Attenuators, Acoustic Lining & Anti-Vibration Connectors for Ductwork
Ducted air-conditioning solves the temperature problem - but creates a new one: noise. The unit's fan, vibrating duct walls, air whistling through bends: all these travel through the ductwork into every room of the building.
The solution isn't to "turn the volume down". The solution involves three specialised techniques: sound attenuators (silencers), internal acoustic lining of ducts, and anti-vibration connectors between unit and ductwork.
1. Sound Attenuators (Silencers): The duct's "muffler"
A sound attenuator looks like a section of duct - but inside it hides panels of sound-absorbing material (usually mineral wool clad with perforated sheet metal). Air passes between the panels, and noise gets "soaked up" by the acoustic walls.
📍 Where to install
Immediately after the discharge (outlet) of the unit, where noise is at its peak. In some installations a second attenuator on the return side is added if the return duct path passes close to bedrooms.
📐 Size and pressure drop
Sound attenuators consume space (60-120 cm in length) and create pressure drop (15-50 Pa). The designer must account for this in the available fan static pressure budget - otherwise airflow in the duct network will be choked.
🔲 Types: Splitter vs Cylindrical
Splitter attenuators (with internal baffles) are used in rectangular ducts for large commercial projects. Cylindrical units fit into round ducts in residential systems - more compact but with somewhat lower attenuation performance.
2. Acoustic Lining: The "velvet" insulation inside the duct
In certain installations, instead of fitting a separate attenuator, we line the interior of the duct with sound-absorbing material (a thin layer of mineral wool or special foam). This is known as Acoustic Lining.
🧹 Internal application
The absorptive material (25-50 mm thick) is glued or mechanically pinned to the inner duct walls. It is covered with a perforated aluminium sheet to prevent fibres from being stripped by the airflow.
📊 Effectiveness
Acoustic lining reduces noise by 3-10 dB per metre of duct - particularly effective at mid-frequencies (250-2000 Hz), which are the most irritating in residential environments.
⚠️ Reduced cross-section
Internal lining eats into the duct - 25 mm on each side means losing 50 mm from each dimension. The designer must upsize the duct accordingly so that air velocity stays within acceptable limits.
🏥 Hygiene concerns
In hospitals and food-processing plants, internal lining is forbidden (risk of microbial growth). In those cases we use external attenuators only, with smooth internal stainless-steel duct surfaces.
3. Anti-Vibration Connectors: Cutting the path for vibrations
The unit's fan vibrates with every rotation. If the metal duct is bolted directly to the unit, those vibrations "travel" through the entire network - heard in every room as a low-frequency persistent "hum".
🔗 What is it?
A soft piece of canvas (fabric or neoprene), 10-15 cm wide, inserted between the unit's flange and the duct's flange. It acts like a "seismic joint" - breaking the rigid mechanical connection.
🛡️ Supply + return sides
We install an anti-vibration connector on both the supply and the return sides of the unit. Vibration can travel from either direction - it must be "cut" in both paths.
⚠️ Don't over-stretch
The canvas must have a slight slack. If stretched tight, it transmits vibrations as tension rather than absorbing them. Ideally leave 5-10 mm of slack after connection.
🔧 Anti-vibration bases
Beyond the canvas connector, the unit itself should sit on rubber mounts or an independent anti-vibration plinth. Otherwise vibration travels through the floor or masonry, bypassing the connector entirely.
4. Noise limits by building type: What's acceptable where
Each type of space has different noise limits. The design engineer must select the right sound-control components so that noise at diffusers doesn't exceed the acceptable levels.
🏠 Residential: 25-35 dB(A)
Bedrooms require noise below 25 dB(A) - practically inaudible. Living rooms tolerate up to 35 dB(A) thanks to ambient background noise from TVs and kitchens. The right attenuator plus low duct velocities achieve these levels.
🏢 Offices: 35-40 dB(A)
In open-plan offices, the "base noise" (conversations, phones) is already 45-50 dB(A). HVAC just needs to stay below 40 dB(A). In conference rooms, the target drops to 30 dB(A) maximum.
🏥 Hospitals: 20-30 dB(A)
Operating theatres, recovery wards and ICUs demand exceptionally low noise (20-25 dB(A)). This requires large attenuators, low air velocities, and plant rooms positioned far from patient areas.
🎭 Theatres & studios: <20 dB(A)
The most demanding environments. They require NC 15-20 (near-total silence). Multiple attenuators in series, oversized ducts, anti-vibration bases and isolated mechanical rooms are all essential.
💡 Noise is not "normal" in a properly designed HVAC system. With the right attenuator, anti-vibration connector and correct air velocity, the only sound you'll hear is your own peace and quiet.
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Hydraulic Networks & Energy Distribution
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